Trigger finger is a common disorder of the hand that can cause pain, snapping or locking of the fingers or thumb. The sensation of locking or catching has often been referred to as triggering.
Tendons in the hand are strong cords that connect the muscle of the forearm to the bones of the fingers and thumb allowing them to bend the digits into a fist, as well as to straighten. Tendons are covered with a thin layer of tissue called tenosynovium which helps them glide through a protective tunnel called the tendon sheath. (Fig. 1) Certain areas of the tendon sheath are thickened forming specialized bands called pulleys which hold the tendon next to the bone, much like the eyes of a fishing rod hold the line near the rod.
Thickening of the tenosynovium or the tendon itself prevents the tendon from gliding freely within the tendon sheath. When the normal smooth gliding property of the tendon is lost, the digit becomes painful. As the tendon becomes larger, it can either become completely stuck inside the sheath preventing flexion of the digit into a fist or blocked from entering the sheath which may make it impossible to straighten the digit. (Fig. 2) This is often experienced by patients on awakening and may require the use of the other hand to pull the finger into a straightened position.